The Exploration of Neptune：A Noble Gas and Volatile Perspective

Most of the probes visiting other bodies in our Solar system only focused, due to technical shortcomings, on the exploration of the closer planets and planetary bodies and/or their natural satellites, i.e. Mercury, Venus, the Moon, Mars, and Jupiter, the comet 67P/Churyumov-Gerasimenko, or the 25143 Itokawa near-Earth asteroid. At present time, no specific missions to one of the two ice giants of our Solar System, Uranus and Neptune, has been planned. Our knowledge of Uranus and Neptune is, therefore, so far restricted to the data which have been collected during the flyby of the Voyager 2 mission, in January 1986 and August 1989, respectively, and to observations with the Hubble Space Telescope and the Keck Telescope. Ice giants are, in our galaxy, thought to be much more abundant than gas giant planets such as Jupiter or Saturn, therefore a better knowledge of ice giants is essential for our understanding of exoplanet candidates. Among other scientific goals, the atmospheric composition of ice giants, with a particular emphasis on their noble gas and volatile distribution, is of great significance, and can constrain models about their formation and evolution. In this review, we report, in a first part, the volatile inventories and the measurements in the planetary bodies of our Solar System; in a second part, we will discuss the scientific background about the concentration, distribution, and evolution of noble gases and volatiles in Uranus and Neptune, and finally describe a possible scenario of a future interstellar probe visiting one of the two ice giants as well as the feasibility of such a space mission, in term of payloads selection and mission profile. We will as well briefly evoke the possibility of using an ion trap mass spectrometer, a potential payload for the ice giant atmospheric exploration, onboard a Chinese interstellar mission to the outer Solar system.